This invention relates to a computer aided design system for designing a logic circuit comprising a plurality of functional blocks connected through path nets to one another. More particularly, this invention relates to a placement method for placing the functional blocks on a substantially plane area in response to logic connection information indicative of a connection between the functional blocks under a circuit constraint.
In a computer aided design system, it is necessary to place functional blocks of a logic circuit on a substantially plane area. Each of the functional blocks is a logic functional unit such as a gate, a flip-flop, or the like. The functional blocks are connected through path nets to one another. The logic circuit may be a large-scale integrated circuit (LSI) or a very large-scale integrated circuit (VLSI). The substantially plane area may be a chip or a printed circuit board.
Such a placement method is disclosed in an article contributed by Gary Robson to VLSI DESIGN, April, 1984, pages 35 to 43, under the title of "Automatic Placement and Routing of Gate Arrays". The placement method comprises an initial placement step and an iterative improvement step. At the initial placement step, the functional blocks are initially placed on the substantially plane area to provide an initial placement result. At the iteractive improvement step, the initial placement result is iteratively improved in order to improve both its speed and the routability of its product. The path nets are divided into critical or crucial nets and normal nets. The critical nets are considered more important in the overall substantially plane area than the normal nets. The critical nets connect those parts of the functional blocks which are called critical blocks. It is necessary to satisfy a circuit constraint with regard to the critical nets. The circuit constraint is, for example, dependent on path lengths and/or path crossovers. In order to provide a final placement result so that the circuit constraint is satisfied, placement of the critical blocks is improved with the critical nets weighted.
As mentioned above, all of the functional blocks are initially placed on the substantially plane area to provide the initial placement result at the initial placement step of the conventional placement method no matter whether the functional blocks are the critical blocks or not. Accordingly, it is difficult to emphasize a critical connection under the circuit constraint. As a result, the conventional placement method is disadvantageous in that it is difficult to provide the final placement result so that the circuit constraint is satisfied despite the fact that the critical nets are weighted. Such a difficulty has lately become more and more evident with large scale integration of the logic circuits such as the LSI.